Bright light emission from silicon quantum dots in a cuvette. The image is from a camera that captures the near-infrared light that the quantum dots emit. The light emission shown is a psuedo color, as near-infrared light does not fall in the visible spectrum. Credit: Folarin Erogbogbo

In a new study, the crystals had no toxic effects in non-human primates

“This is the first test of these silicon quantum dots in primates, and the research results mark a step forward toward potential clinical applications.”

Paras Prasad, SUNY Distinguished Professor in chemistry, physics, electrical engineering and medicine, and executive director of the Institute for Lasers, Photonics and Biophotonics

University at Buffalo

Download High-Res Image

Bright light emission from silicon quantum dots in a cuvette.
The image is from a camera that captures the near-infrared light
that the quantum dots emit. The light emission shown is a psuedo
color, as near-infrared light does not fall in the visible
spectrum. Credit: Folarin Erogbogbo

BUFFALO, N.Y. — Tiny silicon crystals caused
no health problems in monkeys three months after large doses were
injected, marking a step forward in the quest to bring such
materials into clinics as biomedical imaging agents, according to a
new study.

The findings, published
online July 10 in the journal ACS Nano, suggest that the
silicon nanocrystals, known as quantum dots, may be a safe tool for
diagnostic imaging in humans. The nanocrystals absorb and emit
light in the near-infrared part of the spectrum, a quality that
makes them ideal for seeing deeper into tissue than traditional
fluorescence-based techniques.

“Quantum dots, or nanocrystals, are very, very promising
for biomedical imaging applications, but everyone’s worried
about the toxicity and what will happen to them if they
degrade,” said co-lead author Folarin Erogbogbo, PhD, a
University at Buffalo research assistant professor who has since
accepted a new position as an assistant professor of biomedical
engineering at San Jose State University. “Silicon
nanocrystals can be the solution to that because they don’t
contain materials like cadmium that are found in other quantum
dots, and are generally considered to be nontoxic.”

The study was a collaboration between UB, Chinese PLA General
Hospital in China, San Jose State University, Nanyang Technological
University in Singapore and Korea University in South Korea.
It’s part of a larger body of research that many of the team
members have been conducting to investigate the effect of various
nanoparticles in animal models.

The researchers tested the silicon quantum dots in rhesus
macaques and mice, injecting each animal with 200 milligrams of the
particles per kilogram of the animal’s weight.

Blood tests taken for three months afterward showed no signs of
toxicity in either the mice or monkeys, and all of the animals
appeared healthy over the course of the study. The subjects ate,
drank, groomed, explored and urinated normally.

The silicon crystals did, however, gather and stay in the livers
and spleens of the mice, resulting in side effects including
inflammation and spotty death of liver cells.

Interestingly, the same thing did not happen with the rhesus
macaques: The monkeys’ organs appeared normal, without the
damage seen in the mice.

This discrepancy raises the question of how useful toxicity
studies on mice can be in determining a nanocrystal’s
potential effect on humans, said co-author Paras Prasad, PhD, SUNY
Distinguished Professor in chemistry, physics, electrical
engineering and medicine at UB, and executive director of
UB’s Institute for Lasers, Photonics and Biophotonics.

Quantum dots and other nanoparticles — because of their
tiny size — can access parts of the body where larger
particles just can’t go. Due to this and other factors, the
differences in anatomic scale between mice and primates may matter
more in nanomedicine than in other pharmaceutical fields, Prasad
said.

“Even at high doses, we didn’t see any adverse side
effects at all in monkeys despite the problems in mice,”
Prasad said. “This is the first test of these silicon quantum
dots in primates, and the research results mark a step forward
toward potential clinical applications.”

The fact that the silicon did not biodegrade in the mice was
very surprising, said co-author Mark Swihart, PhD, a UB professor
of chemical and biological engineer and co-director of UB’s
New York State Center of Excellence in Materials Informatics.

“Generally, people assume that silicon quantum dots will
biodegrade,” Swihart said. “We didn’t see that
happen, and we think this might be due to the fact that we capped
the surface with organic, FDA-approved molecules to keep the
quantum dots from degrading too fast.

“We may have done too good of a job of protecting
them,” Swihart continued. “If you really kept your car
beautifully waxed all the time, it would never rust. That’s
what we’ve done with these quantum dots.”